CHICAGO — Three former executives of a film-recovery plant were found guilty of murder Friday in a landmark trial stemming from the death of a worker who inhaled cyanide fumes on the job.

Judge Ronald Banks, who presided over the bench trial in Cook County Circuit Court, issued the verdicts in what is believed to be the nation's first case in which corporate officials were charged with murder in the job- related death of an employee.

Those found guilty are Steven J. O'Neil, president of the defunct Film Recovery Systems Inc. of Elk Grove Village, a Chicago suburb; Charles Kirschbaum, the plant manager; and Daniel Rodriguez, the plant foreman. They also were found guilty on 14 counts of reckless conduct.

The three are to be sentenced June 28 and face at least 20 years in prison. Thomas Royce, a lawyer for two of the defendants, called the decision a landmark and said it was ''very, very significant.'' He said it sent a message that corporations would be held accountable to the criminal justice system.

The case stemmed from the death on Feb. 10, 1983, of Stefan Golab, 61, a Polish immigrant employed at the plant. Golab worked over tanks of cyanide solution used in the plant's recovery of silver from used photographic film.

The judge issued findings about evidence presented during the case, including evidence that Golab died of acute cyanide poisoning.

Banks said that although there was no evidence indicating the amount of cyanide in the air in the plant, testimony from a police officer about air quality the day the worker died convinced him that cyanide fumes were present. According to scientific findings, the symptoms were those that would occur if workers were exposed to cyanide, he said.

''I found the conditions in which the workers performed their duties were totally unsafe,'' the judge said.

He said that although workers did not know of the peril in which they worked, company officials were well aware of the hazards.

The three defendants and two other company officials had been indicted Oct. 15, 1983, by a Cook County grand jury.

In finding the three men guilty, Banks also found that their company, Film Recovery, and Metallic Marketing Systems Inc., which owned half the Elk Grove Village plant, both were guilty of involuntary manslaughter and 14 counts of reckless conduct.

On May 14 Banks dismissed all charges against defendant Gerald Pett, the plant's vice president, saying prosecutors had not proved any of the charges against him. Another official, Michael T. MacKay of Salt Lake City, was indicted, but former Utah Gov. Scott Matheson refused to extradite him to Illinois to stand trial.

How Golab died and whether plant officials knew there were life- threatening conditions in the plant were the key issues during the eight- week trial.

Prosecutors charged that Golab inhaled a lethal dose of cyanide fumes as he prepared to disconnect a pump used to clean the 1,000-gallon tanks at the factory. He staggered into a plant lunchroom, began shaking violently and lapsed into unconsciousness, witnesses said.

Defense attorneys, however, claimed Golab died of a heart attack unrelated to his work.

The main prosecution witness, Cook County medical examiner Robert Stein, testified that Golab had a lethal level of cyanide in his blood. He said that when he performed Golab's autopsy, a smell of bitter almonds, the odor associated with the poison, spread throughout the examining room.

Under the state's murder statute, prosecutors had to prove that plant officials ignored the conditions despite knowing there was a risk of harm to a worker.

Nearly 20 of the plant's former workers, all but one of whom were illegal aliens, testified that they vomited almost daily because of the plant's fumes. Many of the former workers said they knew they were working with poison but that they were not told it was cyanide or that the fumes could be deadly.